"text-align: center">MySQL切分函数substring()的具体使用

一、LEFT() 函数

"htmlcode">

mysql> SELECT LEFT('www.csdn.net',5) from web_info w;
+------------------+
| LEFT('www.csdn.net',5) |
+------------------+
| www.c   |
+------------------+
1 row in set (0.00 sec)

二、RIGHT() 函数

"htmlcode">

mysql> SELECT RIGHT('www.csdn.net',5) from web_info w;
+-------------------------+
| RIGHT('www.csdn.net',5) |
+-------------------------+
| n.net     |
+-------------------------+
1 row in set (0.00 sec)

三、SUBSTRING() 函数

substring()是专门用来对字符串进行切分的函数,主要有两种形式:

  • SUBSTRING(string,position);
  • SUBSTRING(string,position,length);

以下是上述两类语句的SQL标准版本写法,意义相同,它更长,但更具表现力。

SUBSTRING(string FROM position);
SUBSTRING(string FROM position FOR length);

下面让我们来一起看一下SUBSTRING()的两种用法示例;

1.SUBSTRING(string,position)

  • string:参数是要操作的字符串。
  • position: 参数是一个"整数",用于指定子串的起始字符,position可以是正整数或负整数。若position大于操作字符串的长度,则返回空字符串。

例如,从“www.csdn.net”字符串中获取子字符串:“csdn.net”,子串的位置必须从5开始,如以下SELECT语句:

mysql> SELECT substring('www.csdn.net',5) from web_info w;
+-----------------------------+
| substring('www.csdn.net',5) |
+-----------------------------+
| csdn.net     |
+-----------------------------+
1 row in set (0.00 sec)

2.SUBSTRING(string,position,length)

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mysql> SELECT substring('www.csdn.net',5,4) from web_info w;
+-------------------------------+
| substring('www.csdn.net',5,4) |
+-------------------------------+
| csdn       |
+-------------------------------+
1 row in set (0.00 sec)

或者通过配置position,从后往前数;SQL如下:

mysql> SELECT substring('www.csdn.net',-8,4) from web_info w;
+--------------------------------+
| substring('www.csdn.net',-8,4) |
+--------------------------------+
| csdn       |
+--------------------------------+
1 row in set (0.00 sec)

又或者通过SQL标准方式来写,SQL如下:

mysql> SELECT substring('www.csdn.net' FROM 5 FOR 4) from web_info w;
+----------------------------------------+
| substring('www.csdn.net' FROM 5 FOR 4) |
+----------------------------------------+
| csdn         |
+----------------------------------------+
1 row in set (0.00 sec)

另外,MySQL中的 mid(), substr() 等价于 substring() 函数哦!

四、SUBSTRING_INDEX() 函数

"delim"来截取子串的函数,我们日常使用频率是很高的;

  • delim:通过该标识符来进行截取的,delim可以为任意字符,不要为空;
  • count:代表第几次出现;count为正数,代表取标识符出现第count次之前的子串;负数则相反,取标识符出现第count次之后的子串。

例如:‘www.csdn.net',获取第一次出现标识符'.'前面的子串,SQL如下;

mysql> SELECT SUBSTRING_INDEX('www.csdn.net','.',1) from web_info w;
+---------------------------------------+
| SUBSTRING_INDEX('www.csdn.net','.',1) |
+---------------------------------------+
| www         |
+---------------------------------------+
1 row in set (0.00 sec)

获取第一次出现标识符'.'后面的子串,SQL如下;

mysql> SELECT SUBSTRING_INDEX('www.csdn.net','.',-2) from web_info w;
+----------------------------------------+
| SUBSTRING_INDEX('www.csdn.net','.',-2) |
+----------------------------------------+
| csdn.net        |
+----------------------------------------+
1 row in set (0.00 sec)

想获取两个'.'中间的呢?可以嵌套一下,SQL如下;

mysql> SELECT SUBSTRING_INDEX(substring_index('www.csdn.net','.',2),'.',-1) from web_info w;
+---------------------------------------------------------------+
| SUBSTRING_INDEX(substring_index('www.csdn.net','.',2),'.',-1) |
+---------------------------------------------------------------+
| csdn               |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

MySQL切分函数substring()的具体使用

五、实战操作

我们以表web_info为测试表,数据如下;

mysql> select * from web_info;
+------+--------+----------------+---------------------+
| w_id | w_name | w_domain  | pub_time   |
+------+--------+----------------+---------------------+
| 1 | CSDN | www.csdn.net | 2020-09-03 11:29:29 |
| 5 | 百度 | www.baidu.com | 2020-09-18 14:37:38 |
| 6 | 淘宝 | www.taobao.com | 2020-09-03 14:37:57 |
+------+--------+----------------+---------------------+
3 rows in set (0.00 sec)

需求1:获取web_info数据的一级域名;
注:这里仅用于举例说明,与实际会有些出入,如.com.cn那种多级后缀就需另外处理。

mysql> SELECT SUBSTRING_INDEX(w_domain,'.',-2),w.* from web_info w;
+----------------------------------+------+--------+----------------+---------------------+
| SUBSTRING_INDEX(w_domain,'.',-2) | w_id | w_name | w_domain  | pub_time   |
+----------------------------------+------+--------+----------------+---------------------+
| csdn.net       | 1 | CSDN | www.csdn.net | 2020-09-03 11:29:29 |
| baidu.com      | 5 | 百度 | www.baidu.com | 2020-09-18 14:37:38 |
| taobao.com      | 6 | 淘宝 | www.taobao.com | 2020-09-03 14:37:57 |
+----------------------------------+------+--------+----------------+---------------------+
3 rows in set (0.00 sec)

需求2:把w_domain字段数据都改成一级域名;

update web_info set w_domain = SUBSTRING_INDEX(w_domain,'.',-2) ;

修改后查询:

mysql> SELECT * from web_info;
+------+--------+------------+---------------------+
| w_id | w_name | w_domain | pub_time   |
+------+--------+------------+---------------------+
| 1 | CSDN | csdn.net | 2020-09-03 14:54:59 |
| 5 | 百度 | baidu.com | 2020-09-03 14:54:59 |
| 6 | 淘宝 | taobao.com | 2020-09-03 14:54:59 |
+------+--------+------------+---------------------+
3 rows in set (0.00 sec)

好了,MySQL的字符串切分函数今天就说到这儿,如果对小伙伴儿有用,请不要白嫖哦~~

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